PROJECT ABSTRACT Obesity is nearly twice as prevalent in people with serious mental illness (SMI) compared to the general population and is associated with cardiometabolic disorders and a reduced life expectancy of 8 to 32 years in this population. Young adulthood, ages 19-30, is a critical window for interventions to reduce this early mortality health disparity as young adults experience the greatest rate of weight gain of any age group. Effective behavior change interventions to reduce cardiometabolic risk factors associated with obesity exist for adults with SMI, but there is a lack of research on health promotion specifically targeting a younger generation of mental health service users. Important differences in the values, attitudes, and preferences for weight loss in younger vs. older adults warrant a tailored approach. Young adults are less likely to be motivated by the health consequences of being overweight and more likely to be motivated by their appearance and social influences on weight loss than middle-aged and older adults. Lifestyle interventions comprised of young adult peer networks that include access to fun and results-oriented exercise classes and use popular technologies are consistent with the values and preferences of this age group. The objective of this R01 application is to evaluate the effectiveness of a scalable group-based lifestyle intervention (PeerFIT) supported by mHealth technology and social media that could be widely adopted to promote weight loss and improved fitness in young adults with SMI. We propose a two-arm randomized controlled trial conducted in real world mental health settings to evaluate the effectiveness of the 12-month PeerFIT intervention compared to Basic Education in fitness and nutrition supported by a wearable Activity Tracking device (BEAT) in achieving clinically significant improvements in weight loss and cardiorespiratory fitness. We will enroll 144 overweight and obese (BMI ? 25) young adults ages 19 to 30 with SMI attending one of two community mental health organizations situated within the Massachusetts Department of Mental Health who are interested in losing weight and improving their fitness. We hypothesize that PeerFIT compared to BEAT will be associated with a greater proportion of participants who achieve cardiovascular risk reduction at 6 and 12 months follow-up as indicated by either clinically significant weight loss (5% or greater) or increased fitness (>50 m on the six- minute walk test). We will also investigate two theoretical mechanisms of action hypothesized to account for greater weight loss and increased cardiorespiratory fitness among participants assigned to PeerFIT: 1) improved self-efficacy and 2) increased peer social support for health behavior change. We hypothesize that the relationship between PeerFIT and weight loss and improved fitness will be mediated by improved self- efficacy and peer support for health behavior change. If proven effective, PeerFit will provide a practical population-based approach to addressing cardiometabolic risk in young adults with SMI within real world community mental health settings.